|Publication number||US7925045 B2|
|Application number||US 11/853,004|
|Publication date||Apr 12, 2011|
|Filing date||Sep 10, 2007|
|Priority date||Sep 10, 2007|
|Also published as||US8170275, US20090067666, US20110149348|
|Publication number||11853004, 853004, US 7925045 B2, US 7925045B2, US-B2-7925045, US7925045 B2, US7925045B2|
|Inventors||Maria Qian Zhao, Wei Ming|
|Original Assignee||Konica Minolta Systems Laboratory, Inc.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (7), Referenced by (2), Classifications (26), Legal Events (3)|
|External Links: USPTO, USPTO Assignment, Espacenet|
1. Field of the Invention
This invention relates to document management, and in particular, it relates to a method for determining the authenticity of a digital document after it has been printed and then scanned in a closed-loop process.
2. Description of Related Art
A closed-loop process refers to printing an original digital document (which may include text, graphics, images, etc.), using the printed hard copy of the document such as distributing it, copying it, etc., and then scanning a hard copy of the document back into digital form. Authenticating a scanned digital document refers to determining whether the scanned document is an authentic copy of the original digital document, i.e., whether the document has been altered while it was in the hard copy form. Certain proposed method of authenticating a scanned document performs an image comparison of the scanned document with the original digital document. Such a comparison may be difficult to achieve to adequate accuracy due to various distortions to the original digital image during the print and scan process.
The present invention is directed to a method for authenticating a document that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to detect alterations to documents in a closed-loop process, i.e., after the document is print and then scanned.
A document authentication method according to embodiments of the present invention eliminates the sole reliance on the method of original digital image and scanned image comparison when detecting image authenticity.
Additional features and advantages of the invention will be set forth in the descriptions that follow and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims thereof as well as the appended drawings.
To achieve these and/or other objects, as embodied and broadly described, the present invention provides a method for authenticating a printed document, including: obtaining a digital document image; adding a watermark to the digital document image, the watermark including a pattern of dots, each dot being larger than or equal to one by one pixel as defined by a first spatial resolution; printing the document including the watermark using halftone printing at the first spatial resolution; scanning the printed document at a second spatial resolution equal to or higher than the first spatial resolution to obtain a scanned image; and detecting any alteration in the scanned image by identifying areas within the scanned image containing low pixel intensity variation.
In another aspect, the present invention provides a method for authenticating a printed document, the document having been printed from a digital image using halftone printing at a first spatial resolution, the digital image containing a watermark added to an original digital document image, the watermark including a pattern of dots, each dot being larger than or equal to one by one pixel as defined by the first spatial resolution, the method including: scanning the printed document at a second spatial resolution equal to or higher than the first spatial resolution to obtain a scanned image; and detecting any alteration in the scanned image by identifying areas within the scanned image containing low pixel intensity variation.
In another aspect, the present invention provides a computer program product that causes a data processing apparatus to perform the above methods.
In yet another aspect, the present invention provides a data processing system, which includes: a scanning section for scanning a printed document to generate a scanned image, the printed document having been printed from a digital image using halftone printing at a first spatial resolution, the digital image containing a watermark added to an original digital document image, the watermark including a pattern of dots being larger than or equal to one by one pixel as defined by the first spatial resolution, wherein the scanner scans the printed document at a second spatial resolution equal to or higher than the first spatial resolution; and a processing section for processing the scanned image to detecting any alteration therein by identifying areas within the scanned image containing low pixel intensity variation.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
A document authentication method according to embodiments of the present invention uses a watermark that is added to the digital image as well as printer halftone information to accomplish detection of alterations in the printed document. The overall process of the authentication method is shown in
In the following descriptions, unless otherwise specified, an 8-bit grayscale range where 0 is black is used in the various examples. Those skilled in the art will be readily able to apply the methods to a different grayscale range.
When adding the watermark to the original digital document, regions of overlap (i.e. where the dots of the watermark exist) are darkened or lightened to reflect the blend color. In a preferred embodiment, when an original pixel value in a region of overlap is lighter than or equal to an intensity threshold value (i.e. it is located in a relatively light colored or blank area of the original image), it is darkened by a value referred to as the first watermark intensity value. When an original pixel value in a region of overlap is darker than the intensity threshold value (i.e. it is located in an area of the image that has relatively dark content), it is lightened by a value referred to as the second watermark intensity value. The first and second watermark intensity value may be the same or different. In one particular example, the intensity threshold value is 255 (i.e. white), and the first and second watermark intensity values are 51. In an alternative embodiment, pixels in all regions of overlap are darkened regardless of the original pixel value (resulting pixel values exceeding the darkest possible value (0) are set to the darkest pixel value). More generally, the watermark intensity values added or subtracted from the original pixel value may be any suitable function of the original pixel value, with the general goal of generating an image with visible watermark dots in all image areas (both originally dark areas and originally light areas). In addition, the size of the watermark dots may vary depending on the original pixel value. For example, watermark does in very dark areas (lighter toned dots in vary dark areas) may be made larger than gray toned watermark dots in white areas.
This watermarked image is printed out as a halftone image at the first spatial resolution (e.g. 300 dpi) (step S13), and the printed document is used (e.g. distributed, etc.) The printed document carries a visible watermark in the form of a light gray shade.
Later, a detection process shown in
The alteration detection algorithm used in step S15 is described below with reference to the flowchart shown in
Steps S305 to S314 are explained with reference to the example shown in
For each perspective candidate for alteration, an extended block of interest for which the determined centroid acts as its center is selected (step S309). In a preferred embodiment, a size of 32×32 pixels is used for the extended block of interest.
Referring back to
In preferred embodiments, the resolution at which the watermarked document is scanned back is higher than the resolution at which the watermarked document is printed. Alternatively, the scan resolution may be the same as the print resolution, but it would require the original digital image intensity to be lightened or the original image resolution to be reduced. By doing so, the halftone dots can still be visible even if not scanned at a higher resolution. Such measures, however, tend to reduce the image quality of the printed document and are not preferred.
The alteration detection method described above can be applied even when the printed document is copied before it is altered, as long as the copying preserves the halftone nature of the printed document or is itself a halftone image generated by the printer. If the printed document is copied after it is altered, the applicability of the alternation detection method would depend on the way the copier manipulates the image when copying.
It will be apparent to those skilled in the art that various modification and variations can be made in the method for detecting alterations in a printed document according to the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover modifications and variations that come within the scope of the appended claims and their equivalents.
|Cited Patent||Filing date||Publication date||Applicant||Title|
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|US7142689 *||Jan 30, 2001||Nov 28, 2006||Canon Kabushiki Kaisha||Image processing apparatus for determining specific images|
|US7286685 *||Jan 31, 2006||Oct 23, 2007||Digimarc Corporation||Halftone watermarking and related applications|
|US7586647 *||Jul 6, 2005||Sep 8, 2009||Canon Kabushiki Kaisha||Halftone detection and removal|
|US7644281 *||Sep 27, 2004||Jan 5, 2010||Universite De Geneve||Character and vector graphics watermark for structured electronic documents security|
|US20060023259||Jul 6, 2005||Feb 2, 2006||Canon Kabushiki Kaisha||Halftone detection and removal|
|US20060075241 *||Sep 27, 2004||Apr 6, 2006||Frederic Deguillaume||Character and vector graphics watermark for structured electronic documents security|
|Citing Patent||Filing date||Publication date||Applicant||Title|
|US8634110 *||Oct 6, 2010||Jan 21, 2014||Hewlett-Packard Development Company, L.P.||Edge refinement system|
|US20120086984 *||Oct 6, 2010||Apr 12, 2012||Robert Alan Ulichney||edge refinement system|
|U.S. Classification||382/100, 382/274, 713/176|
|International Classification||H04L9/32, G06K9/00|
|Cooperative Classification||H04N2201/3233, H04N2201/3235, G06T2201/0061, G06T2201/0083, H04N2201/3269, G06T1/0042, G06T1/0078, H04N1/32325, H04N1/32203, H04N2201/3271, G06K9/6202, H04N1/32293, H04N1/32208, G06T2201/0051|
|European Classification||H04N1/32C19B3, G06T1/00W6T, G06T1/00W4, G06K9/62A1, H04N1/32C19B9B, H04N1/32C19B6B, H04N1/32C19B3B|
|Sep 10, 2007||AS||Assignment|
Owner name: KONICA MINOLTA SYSTEMS LABORATORY, INC., CALIFORNI
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:ZHAO, MARIA;MING, WEI;REEL/FRAME:019804/0866
Effective date: 20070910
|Jul 15, 2011||AS||Assignment|
Effective date: 20101231
Owner name: KONICA MINOLTA LABORATORY U.S.A., INC., CALIFORNIA
Free format text: MERGER;ASSIGNOR:KONICA MINOLTA SYSTEMS LABORATORY, INC.;REEL/FRAME:026602/0253
|Sep 10, 2014||FPAY||Fee payment|
Year of fee payment: 4